The biosynthesis of ACTH, endorphin, Alpha-MSH, vasopressin and oxytocin, was studied, with emphasis on the enzymes involved in the proteolytic processing of the respective prohormones. A prohormone converting enzyme (PCE) which specifically cleaves at paired basic residues of the endogenous prohormones (pro-opiomelanocortin, pro-oxytocin and pro-vasopressin) to the active hormones, has been purified from bovine pituitary intermediate and neural lobe secretory vesicles. Purified PCE from both lobes appears to be identical and has been characterized as a approximately 70,000 dalton glycoprotein. PCE exists in a soluble and membrane associated form and was found to cleave pro-insulin to insulin as well. A carboxypeptidase B-like enzyme and an aminopeptidase B-like enzyme which function to remove the basic residues from the C- and N-terminal, respectively, from the peptide hormone, following the action of PCE, have been found in intermediate lobe and neural lobe secretory vesicles. The aminopeptidase B-like enzyme has been partially purified and characterized as a greater than 75,000 dalton metallopeptidase. The regulation of biosynthesis of pro-opiomelanocortin (POMC) in the toad pituitary intermediate lobe by dopamine and cAMP was also studied. Pharmacological analysis indicates that the toad intermediate lobe dopamine receptor is of the D2 category and negatively coupled to adenylate cyclase. Thus the dopamine acts, subsequent to binding to the receptor, by lowering the intracellular cAMP level which then results in a decrease in POMC synthesis in the tissue.